Electroabsorption modulator having a barrier inside a quantum well

ABSTRACT

An electroabsorption modulator ( 10 ) includes at least one quantum well ( 26 ) in a conduction band and a corresponding quantum well ( 26 ) in a valence band. A barrier made from InGaAlAs or InGaAsP is formed within each of the quantum wells in the conduction and valence bands.

TECHNICAL FIELD

[0001] The present invention generally relates to electroabsorptionmodulators, and more particularly to a high-power electroabsorptionmodulator having a barrier inside quantum wells.

BACKGROUND ART

[0002] Electroabsorption modulators (EAMs) have been shown to be usefulin fiber-optic links operating near either 1.3 or 1.55 μm for bothanalog and digital signal transmission. They are small in size and canbe integrated with a laser diode, for example, while being veryeffective in changing the light intensity as a function of the appliedelectric field. In particular, EAMs that use multiple quantum wellactive regions are currently very popular. As those skilled in the artwill understand, a quantum well is a material unit that has a very thin“well” material (typically of the order of 100 Angstrom) surrounded bypotential energy barriers. Inside the quantum well, particle, such aselectrons or holes, are restricted in motion in the dimensionperpendicular to the well and has well-defined quantized states withrespect to momentum and energy. The particles are free to move around inthe other two directions. When these quantum wells are stacked togetherwithout interactions between each other, this resultant structure isreferred to as multiple quantum wells (MQW). The MQW provides a largeabsorption-coefficient change via the quantum-confined Stark effect(QCSE), which is the shift of the excitonic (the resonance state ofelectron-hole pairs) absorption peak of the quantum well under theinfluence of an applied electric field. Typically, the peak shifts tolower energy when an electric field is applied.

[0003] An analog fiber link is an optical fiber communication channelfor transmitting analog signals. For an externally modulated analogfiber link, which uses a transmitter with the light modulation occurringoutside the laser source, increasing the received optical power reducesthe link loss, which is the ratio of the input signal intensity to theoutput signal intensity. The output signal of the receiver isproportional to the square of the optical power. The optical power usedin the link, however, is currently limited by the optical saturationproperties of the EAM. An optical saturation occurs when the outputsignal intensity is no longer linearly proportional (or becomessub-linear) to the input signal power to the EAM. Consequently, aconcern for the MQW EAMs is their relatively low saturation opticalpower, where optical saturation begins. This power level can bedetermined from the optical power when the output signal intensity is 1dB below that which corresponds to unsaturated modulation. Theconventional MQW EAMs, particularly those made of InGaAs/InP, tend tosaturate at a much lower level.

[0004] In quantum wells (QWs), the barriers hinder the sweep-out of thephoto generated electrons and holes, particularly holes, resulting incarrier pile-up near the barriers. The traditional approach to reducethis effect had been to use InGaAsP or InAlAs (or InGaAlAs) instead ofInP as barrier materials to reduce the valence band offset, which wasshown to improve the optical saturation of the MQW EAMs. Also, therehave been attempts to use strain-compensated InGaAsP/InGaAsP andInAsP/GaInP quantum wells, which have shallow wells, to improve thesaturation optical power at 1.55 and 1.3 μm. A strained quantum wellrefers to a quantum well where either the barrier or the quantum well islattice mismatched to the substrate so that the quantum well as a unitis under (tensile or compressive) hydrostatic strain. A straincompensated quantum well refers to a quantum well structure in which thewell and the barrier are oppositely (compressive versus tensile)strained so that the net strain in one unit of quantum well is zero.Although it has been reported that the MQW EAMs with InGaAs/InAlAs canhandle optical power up to 40 mW without degradation in the bandwidth,the link gain at RF frequencies was observed to saturate at a much lowerlevel. The maximum optical power that does not cause RF gain saturationis currently limited to approximately 10 mW.

[0005] It has also been observed that increasing the electric fieldreduces the screening effect due to spatially distributed holes thatcannot be drifted out of the quantum well (their presence causes aneffective reduction of the applied electric field). Hence in order toincrease the saturation optical power further, the operating bias mustbe increased for a given intrinsic layer thickness without compromisingthe modulator performance such as modulator slope efficiency, which isdefined as the maximum change in optical transmission versus change inapplied voltage.

DISCLOSURE OF INVENTION

[0006] One embodiment of the invention is directed to anelectroabsorption modulator including first and second electricallyconductive electrodes, and first and second cladding layers providedbetween the first and second electrodes. A quantum well layer isprovided between the first and second cladding layers, and includes anInAlAs first region, an InGaAs second region adjacent the first region,and an InGaAlAs third region adjacent the second region.

[0007] In accordance with another embodiment of the present invention,an electroabsorption modulator includes first and second electricallyconductive electrodes, and first and second cladding layers providedbetween the first and second electrodes. A multiple quantum well layeris provided between the first and second cladding layers, and eachquantum well includes an InP (or InGaAsP) first region, an InGaAs (orInGaAsP) second region adjacent the first region, and an InGaAsP thirdregion adjacent the second region.

[0008] Another embodiment of the invention is directed to anelectroabsorption modulator having at least one quantum well in aconduction band and a corresponding quantum well in a valence band. Astep barrier made from InGaAlAs or InGaAsP is formed within the quantumwells of both the conduction and valence bands.

[0009] A further embodiment of the present invention relates to a methodfor delaying a red shift of a quantum-confined Stark effect to a higherelectric field, and increasing transition energy and an oscillatorstrength response after the onset of the red shift, in anelectroabsorption modulator. The method includes creating at least onefirst quantum well in a conduction band and at least one correspondingsecond quantum well in a valence band, and forming a step barrier madefrom InGaAlAs or InGaAsP in a portion of the first and second quantumwells.

BRIEF DESCRIPTION OF DRAWINGS

[0010]FIG. 1 is an electroabsorption modulator in accordance with anembodiment of the present invention;

[0011]FIG. 2 is an energy band diagram of the electroabsorption regionof the modulator of FIG. 1 at zero electric field;

[0012]FIG. 3 is an energy band diagram of the electroabsorption regionof the modulator of FIG. 1 at electric field of approximately 100 kV/cm;

[0013]FIG. 4 is an energy band diagram of the electroabsorption regionof the modulator of FIG. 1 at electric field of approximately 140 kV/cm;

[0014]FIG. 5 is an energy band diagram of the electroabsorption regionof the modulator of FIG. 1 at electric field of approximately 200 kV/cm;

[0015]FIG. 6 is a graph showing the transition energy shift as afunction of the electric field for the electroabsorption modulator ofFIG. 1, compared with conventional electroabsorption modulators;

[0016]FIG. 7 is a graph showing the square of the overlap integral as afunction of the electric field for the electroabsorption modulator ofFIG. 1, compared with conventional electroabsorption modulators;

[0017]FIG. 8 is a graph showing the normalized transmission of theelectroabsorption modulator of FIG. 1, compared with conventionalelectroabsorption modulators;

[0018]FIG. 9 is an electroabsorption modulator in accordance withanother embodiment of the present invention; and

[0019]FIG. 10 is an energy band diagram of the electroabsorption regionof the modulator of FIG. 9.

BEST MODE OF CARRYING OUT THE INVENTION

[0020] Turning now to FIG. 1, an electroabsorption modulator (EAM) inaccordance with an embodiment of the invention is indicated generally at10, and includes a n-InP substrate 12 in contact with an n-InAlAswaveguide cladding layer 14 (where n indicates an electronconcentration, for example, of approximately 1×10¹⁸ cm⁻³, in thisembodiment). The EAM 10 also includes an p-InP buffer layer 16 providedbetween a heavily doped p-InGaAs contact layer 18 (where p indicateshole concentration, for example, of approximately 1×10¹⁸ cm⁻³ in thisembodiment) and a p-InAlAs waveguide cladding layer 20 (where p=1×10¹⁸cm⁻³). On the surface opposite the p-InP buffer layer 16, the p-InGaAscontact layer 18 is in contact with a positive electrode 22 made ofelectrically conductive metal alloy such as AuZn followed by gold, forexample. Similarly, the surface of the n-InP substrate 12 opposite thewaveguide cladding layer 14 is in contact with an n type electrode 24,which is also made of electrically conductive metal alloy such as AuGefollowed by gold, for example.

[0021] A multiple intra-step-barrier quantum well (IQW) layer 26 isprovided between the two waveguide cladding layers 14, 20, and includesrepeating regions of an InAlAs barrier 28, an InGaAs well 30 and anInGaAlAs intra-step-barrier 32 (best shown in FIGS. 2-5), in thedirection generally perpendicular to the surfaces of the waveguidecladding layers 14, 20. In one embodiment, the InGaAlAs of the IQW layer22 is grown by repeating InGaAs/InAlAs layers several times to form ashort-period superlattice, and the EAM 10 is grown by molecular beamepitaxy.

[0022] Turning now to FIGS. 2-5, an exemplary energy band diagrams ofthe IQW layer 26 includes the In_(0.52)Al_(0.48)As barrier 28, theIn_(0.53)Ga_(0.47)As well 30 and the In_(0.53)Ga_(0.33)Al_(0.14)Asintra-step-barrier 32 for both conduction (E_(c)) and the valence(E_(v)) bands. The nominal bandgap energy of theIn_(0.53)Ga_(0.33)Al_(0.14)As intra-step-barrier layer 32 is 0.97 eV andthe layer is lattice-matched to InP. It should be noted that theintra-step-barriers 32 in the conduction (E_(c)) and valence (E_(v))bands are generally symmetrical or mirror images of each other, althoughthe depths of the wells 30 and the heights of the step-barriers 32 maydiffer somewhat as a result of the chemical make-up of the wells and thestep-barriers.

[0023] FIGS. 2-5 show the IQW layer 26 with energy levels of a firstelectron state (E1) and a first heavy hole state (HH1), and theelectrons and hole envelope wave functions 34, 36 for a total wellregion 38 (i.e., the well 30 and the intra-step-barrier 32) having awidth of 10 nm, at various electric fields. In this example, the IQWlayer 26 was analyzed with a finite-difference method using the envelopewave functions model under the effective-mass approximation. Only thetransition from the first heavy-hole state (HH1) to the first electronstate (E1) has been considered.

[0024] At zero electric field (FIG. 2), the electrons in the conductionband E_(c) is rather loosely confined over the whole well region 38,while the holes are tightly confined in the well 30 (i.e., the InGaAswell 30) of the valence band E_(v). This is mostly due to the effectivemass difference. As the electric field is applied in the −z direction(e.g., 100 kV/cm) (FIG. 3), the electron envelope wavefunction 34 movesin the +z direction, while the hole envelope wavefunction 36 begins tocross over the intra-step-barrier 32 in the −z direction. Up to thispoint, the overall transition energy shift is very small or even alittle positive (“blue-shifted”). This is because the hole energy levelincreases with the electric field although the electron energy leveldecreases. Hence, the normal “red-shifted” quantum-confined Stark effect(QCSE) is effectively suppressed.

[0025] With the electric field further increased (e.g., 140 kV/cm) (FIG.4), the energy shift becomes negative (red-shifted), as the holeenvelope wavefunction 36 crosses further over the intra-step-barrier 32of the valence band E_(v) and the hole energy level starts to decrease.Also the oscillator strength, which is a measure of the transitionprobability leading to electroabsorption in the quantum well, isproportional to the square of the spatial overlap integral between theelectron and the hole envelope wavefunctions 34, 36, and is reduceddramatically as the hole envelope wavefunction crosses over theintra-step-barrier 32, due to the spatial separation of thewavefunctions. At an even larger electric field, (e.g., 200 kV/cm) (FIG.5), the holes are mostly confined over the intra-step-barrier 32 and theoscillator strength becomes very small.

[0026] The result is summarized in FIG. 6 for the change in thetransition energy, and in FIG. 7 for the square of overlap integral as afunction of the applied electric field. The results are compared withconventional quantum wells (QW) with thickness of 7.2 nm and 10.0 nm.The 7.2 nm QW was chosen as a conventional QW for 1.55 μM operation. The10.0 nm QW was chosen to show the effect of intra-step-barrier, althoughthe zero-field transition energy is not the same as that of the IQW ofthe same thickness.

[0027] In FIG. 6, it can be seen that the intra-step-barrier 32 (bestshown in FIG. 2) effectively suppresses the onset of the red shift ofthe QCSE for up to approximately 100 kV/cm. After this point, thetransition energy decreases quickly, crossing that of a 7.2-nm thickconventional QW with a steeper slope. This implies that the EAM 10 withthe IQW 26 will be more efficient than that of the conventional QW. Whencompared with the conventional QW with thickness of 10.0 nm, the slopeof the energy shift with respect to the electric field is similar afterapproximately 100 kV/cm, the curve for the IQW 26 being translated to ahigher electric field. Hence the IQW 26 effectively takes advantage ofthe wider well region 38 width, with the delayed onset of the red shift.FIG. 7 illustrates a unique feature in the square of the overlapintegral of this IQW 26. This sharp change combined with the energyshift gives a very good QCSE.

[0028] With the above results, the change in the absorption coefficientwith the electric field was estimated for the 7.2-nm thick conventionalquantum well. A Gaussian broadening function was used, whose zero-fieldfull width at half maximum (FWHM) was obtained from experimental values(˜20 me.V). It was varied following the overlap-integral change with theelectric field. The absorption coefficient was also estimated from theexperimental values. The transfer curve as a function of the appliedbias was then calculated for the EAM with intrinsic layer thickness of0.25 μm, an optical confinement factor of 0.20, and a waveguide lengthof 200 μm for 1.55 μm light. The same calculation was repeated for theEAM 10 with the 10.0-nm IQWs with the same device parameters, using thesame form of Gaussian broadening.

[0029] Turning now to FIG. 8, the estimated transfer curves show thatthe present EAM 10 with IQW 26 can be operated at a higher bias for thehighest modulator slope efficiency. For instance, for the FWHM of 20meV, the highest slope efficiency for the IQW EAM 10 (2.9 V⁻¹) occurs atapproximately 2.7 V, while for the conventional QW EAM, it happens atabout 1.4 V, which represents an increase by a factor of approximately2. Moreover, the slope efficiency itself is increased by a factor ofabout 3.6 with the use of the present IQW 26, which is the consequenceof the sharper transition energy shift with the IQW at a higher electricfield. This means that the IQW EAM 10 will not only improve thesaturation optical power, but also yield a higher slope efficiency.

[0030] Referring to FIG. 9, an electroabsorption modulator (EAM) inaccordance with another aspect of the present invention is indicatedgenerally at 40 and includes features similar to those described abovewith respect to the EAM 10 of FIG. 1. The EAM 40 includes a n-InPsubstrate 12′ in contact with an n-InGaAsP waveguide cladding layer 14′(n=1×10¹⁸ cm⁻³). The EAM 40 also includes an n-InP buffer layer 16′provided between a p-InGaAs contact layer 18′ (p=1×10¹⁸ cm⁻³) and ap-InGaAsP waveguide clading layer 20′ (p=1×10¹⁸ cm⁻³). On the surfaceopposite the p-InP buffer layer 16′, the p-InGaAs contact layer 18′ isin contact with a positive electrode 22′ made of electrically conductivemetal alloy such as AuZn followed by gold, for example. Similarly, thesurface of the n-InP substrate 12′ opposite the n-InGaAsP waveguideclading layer 14′ is in contact with a negative electrode 24′, which isalso made of electrically conductive metal alloy such as AuGe, followedby gold, for example.

[0031] A multiple intra-step-barrier quantum well (IQW) layer 42 isprovided between the waveguide cladding layers 14′, 20′, and includesrepeating regions of an InP (or InGaAsP) barrier 44, an InGaAs (orInGaAsP) well 46 and an InGaAsP 48 intra-step-barrier 32 (best shown inFIG. 10), in the direction perpendicular to the surfaces of thewaveguide cladding layers 14′, 20′. In one embodiment, the InGaAsPintra-step-barrier of the IQW layer 42 is grown by repeating InGaAs/InPlayers several times to form a short-period superlattice, and the EAM 40is grown by molecular beam epitaxy or organometallic vapor phaseepitaxy.

[0032] It should be understood that other material systems can also beused to obtain an electroabsorption modulator having anintra-step-barrier quantum well in both the conduction band E_(c) andthe valence band E_(v), with some modification on the band offsets,effective masses, and the composition of the well, barrier, and theintra-step-barrier.

[0033] From the foregoing description, it should be understood that animproved electroabsorption modulator (EAM) has been shown and describedwhich has many desirable attributes and advantages. Theintra-step-barrier quantum well (IQW) moves the onset of the red-shiftedQCSE to a higher electric field. After the onset of the red shift, thetransition energy and the oscillator strength respond sharply to theelectric field. The present EAM with the IQW operates at a higheroptical power with enhanced modulator slope efficiency, which enhancesthe optical link performance.

[0034] While various embodiments of the present invention have beenshown and described, it should be understood that other modifications,substitutions and alternatives are apparent to one of ordinary skill inthe art. Such modifications, substitutions and alternatives can be madewithout departing from the spirit and scope of the invention, whichshould be determined from the appended claims.

[0035] Various features of the invention are set forth in the appendedclaims.

1. An electroabsorption modulator apparatus comprising: first and secondelectrically conductive electrodes; first and second cladding layersprovided between said first and second electrodes; and a quantum welllayer provided between said first and second cladding layers, saidquantum well layer including an InAlAs first region, an InGaAs secondregion adjacent said first region, and an InGaAlAs third region adjacentsaid second region.
 2. The apparatus as defined in claim 1 furtherincluding: a substrate provided between said first electrode and saidfirst cladding layer; a contact layer provided between said secondelectrode and said second cladding layer; and a buffer layer providedbetween said contact layer and said second cladding layer.
 3. Theapparatus as defined in claim 2 wherein said first and second claddinglayers are InAlAs or InGaAlAs, and said substrate is InP.
 4. Theapparatus as defined in claim 3 wherein said second electrode is made ofAuZn and Au, said contact layer is InGaAs, and said buffer layer is InP.5. An electroabsorption modulator apparatus comprising: at least onefirst quantum well in a conduction band; a corresponding second quantumwell in a valence band; a first barrier formed within said first quantumwell; and a second barrier formed within said second quantum well;wherein said first and second barriers are formed from InGaAlAs.
 6. Theapparatus as defined in claim 5 further including: a third barrierdefining one side of said first well in said conduction band; a fourthbarrier defining one side of said second well in said valence band;wherein said first barrier is separated from said third barrier and saidsecond barrier is separated from said fourth barrier, and said first andsecond barriers are symmetrically formed in a lateral direction of saidconduction and valence bands.
 7. The apparatus as defined in claim 6wherein a concentration of electrons in said conduction band aresubstantially over said first well, and a concentration of holes in saidvalance band in said second well is substantially in an area betweensaid second and fourth barriers, when no electric field is applied tosaid apparatus, and wherein said concentration of said electrons shiftsover increasingly to an area between said first and third barriers insaid first well, and said holes increasingly shifts out of said areabetween said second and fourth barriers and over to said second barrier,as electric field is applied to said conduction and valence bands in thedirection substantially perpendicular to said third and fourth barriers.8. The apparatus as defined in claim 6 wherein said third and fourthbarriers are formed from InAlAs, and said first and second wells areformed from InGaAs.
 9. An electroabsorption modulator apparatuscomprising: at least one first quantum well in a conduction band; acorresponding second quantum well in a valence band; a first barrierformed within said first quantum well; and a second barrier formedwithin said second quantum well; wherein said first and second barriersare formed from InGaAsP.
 10. The apparatus as defined in claim 9 furtherincluding a third barrier defining one side of said first well in saidconduction band; a fourth barrier defining one side of said second wellin said valence band; wherein said third and fourth barriers are formedfrom InP or InGaAsP, and said first and second wells are formed fromInGaAs or InGaAsP.
 11. An electroabsorption modulator apparatuscomprising: first and second electrically conductive electrodes; firstand second cladding layers provided between said first and secondelectrodes; and a quantum well layer provided between said first andsecond cladding layers, said well layer including an InP first region,an InGaAs second region adjacent said first region, and an InGaAsP thirdregion adjacent said second region.
 12. The apparatus as defined inclaim 11 further including: a substrate provided between said firstelectrode and said first cladding layer; a contact layer providedbetween said second electrode and said second cladding layer; and abuffer layer provided between said contact layer and said secondcladding layer.
 13. The apparatus as defined in claim 12 wherein saidfirst and second cladding layers are InGaAsP, and said substrate is InP.14. The apparatus as defined in claim 13 wherein said second electrodeis made of AuZn and Au, said contact layer is InGaAs, and said bufferlayer is InP.
 15. A method for delaying a red shift of aquantum-confined Stark effect to a higher electric field, and increasingtransition energy and an oscillator strength response after the onset ofthe red shift, in an electroabsorption modulator, said method comprisingthe steps of: creating at least one first quantum well in a conductionband; creating at least one corresponding second quantum well in avalence band; forming a first barrier in a portion of said first quantumwell; and forming a second barrier in a portion of said second quantumwell; wherein said first and said second barriers are formed fromInGaAlAs.
 16. The method as defined in claim 15 wherein said first andsecond wells are formed from InGaAs or InGaAlAs.
 17. The method asdefined in claim 16 further including the step of creating third andfourth barriers defining each said first and second wells, wherein saidthird and fourth barriers are formed from InAlAs or InGaAlAs.
 18. Amethod for delaying a red shift of a quantum-confined Stark effect to ahigher electric field, and increasing a transition energy and anoscillator strength response after the onset of the red shift, in anelectroabsorption modulator, said method comprising the steps of:creating at least one first quantum well in a conduction band; creatingat least one corresponding second quantum well in a valence band;forming a first barrier in a portion of said first quantum well; andforming a second barrier in a portion of said second quantum well;wherein said first and said second barriers are formed from InGaAsP. 19.The method as defined in claim 18 wherein said first and second wellsare formed from InGaAs or InGaAsP.
 20. The method as defined in claim 19further including the step of creating third and fourth barriersdefining each said first and second wells, wherein said third and fourthbarriers are formed from InP or InGaAsP.